This invention relates to a method for removing turbine buckets from a turbine rotor in a steam turbine.
Dovetail attachments between turbine buckets and turbine rotor wheels are well known and commonly used. Dovetail joints typically comprise a male dovetail on the rim of a rotor wheel and a female dovetail at the base of each turbine bucket to be attached to the wheel. The male dovetail extends annularly around the rim of the wheel and includes a series of annular hooks each extending laterally from wheel. The male dovetail has in cross section a pyramid shape that loosely fits into the female dovetail of each bucket. To tighten the bucket fit on the wheel, the wheel has a lip (xe2x80x9crolledxe2x80x9d) that seats the buckets on the wheel.
The female dovetail has a wide-mouth channel cross-sectional shape, to slide over the male dovetail. The channel forming the female dovetail has xe2x80x9cnecksxe2x80x9d, which are parallel grooves in the sidewall of the channel. The male dovetail has xe2x80x9chooksxe2x80x9d which are annular ridges that extend around the wheel, except for a notch in the wheel. The hooks of the male dovetail fit into necks formed in the female dovetails of the turbine buckets. The notch in the wheel is where turbine buckets are normally inserted and removed from the wheel.
The interlocking male and female dovetails form a loose fit between the turbine buckets and rotor wheel. When subjected to centrifugal forces from the rotating turbine wheel, the dovetail fit is sufficient to firmly hold the turbine buckets during steam turbine operation. When the wheel is stationary, the loose fit allows the female and mail dovetail joints to slide with respect to each other and, thus, enables technicians to easily remove and install buckets from the wheel.
Corroded and damaged turbine buckets must be removed from rotor wheels, so that the buckets can be replaced. Buckets corrode and erode, usually along the length of their blades. These corroded and eroded buckets must be removed from the rotor wheel to be replaced. Typically, all buckets on a specific turbine wheel are exposed to the same conditions and, therefore, corrode and erode at approximately the same rate. Generally, all buckets on a wheel are periodically removed and replaced together.
The loose fit between the dovetail joints can bind, after thousands of hours of operation of the turbine wheel. The dovetail joints bind together due to the high temperatures and pressures at which the buckets and wheels operate, the severe forces that act on the wheel and buckets, and the caustic hot gases that pass over the buckets. These conditions can cause the dovetail joints between the buckets and wheel to bind and form a permanent tight fit. When the dovetail joint binds, it is nearly impossible to slide the bucket over the male dovetail and remove the bucket from the wheel.
In the past, when a bucket became stuck on a wheel, a single cut was made in the base of the bucket, just above the top of the female dovetail. The blade of the turbine bucket was broken off at the cut. Once the blade was removed, one solid mass of the female dovetail portion of the base was left stuck on the wheel. The dovetail from the decapitated bucket and the male dovetail of the wheel were still stuck together.
After the blade was broken off, it was difficult to rotate the female dovetail around the male dovetail so as to remove the female dovetail from the wheel. The application of considerable force was required to slide the remaining female piece around the wheel to a notch from which the dovetail could be removed. This force involved the use of an impact tool, such as an air hammer or a peening gun. The impact of the tool often caused extensive damage to the turbine wheel. Moreover, it was difficult to slide each side of the female dovetail at the same rate around the wheel. When the two sides of the dovetail slide at different rates, the dovetail pinched the wheel and became even harder to remove.
The turbine wheel, which includes the male dovetail, is particularly susceptible to damage when hit by a hammer or other hard tool that is usually used to remove a stuck bucket. The damage to the wheel results in substantial and costly repairs or potential replacement of the male dovetail. Accordingly, there is a long-felt need for a method and system to efficiently remove dovetailed turbine buckets from the turbine wheel while minimizing the risk of damage to the wheel.
A method and system has been developed for efficiently removing turbine buckets that have become bound or stuck to a rotor wheel. The method and system is especially suited to turbine buckets that have dovetails. The method and system involves using a cutting device to cut multiple grooves into the female dovetail of the bucket. The cuts allow the blade of the bucket to be removed, and then the female dovetail to be sectioned. The dovetail sections are removed from the wheel. This method is particularly suited for servicing industrial steam turbines and for use by power system service centers. The method does not damage turbine wheels, but does destroy damaged buckets. The bucket removal method has been used to remove a row of buckets from a wheel in two to three hours, in contrast to the 15 to 31 hours previously required.
This method includes making two or more first cuts to the upper neck of a female dovetail of the turbine blade, and making two or more second cuts adjacent a lower neck of the female dovetail. The blade of the bucket is broken off at the first cuts, which breakage also splits the base of the dovetail into two dovetail sections on opposite sides of the male dovetail portion of the turbine wheel. These dovetail sections are further split along the second cuts. The split sections are removed by directly removing the sections or sliding them around the wheel to a notch, which normally is used to remove buckets.
In particular, the removal method involves cutting, at least, four relief cuts to the female dovetail section of the turbine buckets. The relief cuts are single cuts made horizontally at a neck of the female bucket component. Two upper relief cuts are made on either side of the bucket near the top of the dovetail joint. These two upper cuts are made to a depth just less than completely through the bucket. These upper cuts allow the blade portion to be removed by striking it with a force sufficient to break the blade away from the bucket. After the blade is broken away, two separate pieces of the bucket""s female dovetail are attached to the male dovetail of the wheel. The relief cuts are made such that when the bucket blade is removed, the flat, top portion of the male dovetail wheel is exposed.
Two similar relief cuts are made just above the bottom of the bucket. These cuts are made to relieve the tight fit of the bottom dovetail that resulted during operations from the wheel pressing and rolling against the bucket. Additional relief cuts may be made, as necessary, at locations on the bucket that are bound to the wheel. These cuts relieve any force exerted by the male dovetail holding the bucket piece to the turbine wheel. These lower cuts allow the remaining two halves of the female dovetail to be directly removed from the wheel or slid around the wheel and removed at the wheel notch.